Often in cooking, especially in cases where dairy products are involved, one is faced with either the choice of leaving a pot open, or covering it with a lid. If you choose to leave the pot open, it takes longer for the pot to heat up and when it finally does, the liquid ingredients (water or milk, for example) tend to evaporate. This evaporation thus causes waste of some of the ingredients, not to mention loss of some of the aroma. Often, in this case, you need to add more of these liquid ingredients to compensate for the loss.
The other choice is to leave the lid on the pot to prevent evaporation of the liquid ingredients, which preserves the steam/heat and allows the food to cook faster. However, this choice also has major drawbacks. For example, if left unattended—even for a short period of time—the pot can heat too quickly and the contents can boil over (i.e., ebullition of the liquid ingredients)—creating a mess on the cooktop and the pan, and perhaps ruining the food in the pot itself or, at the very least, causing loss of some of the food.